(1) Field of Invention
The present invention relates to a wireless transmission system and a wireless transmission method in which a plurality of wireless transmission devices transmit and receive signals utilizing a transmission scheme with an anti-multipath property, and relates to a wireless station and a transmitting station used therein.
(2) Description of the Related Art
In the field, of wireless communications, there is a technique utilizing a multipath-resistant modulation/demodulation scheme, in which a signal, is transmitted simultaneously to a plurality of transmitting stations so as to deliberately create a plurality of signal paths, and a plurality of arriving waves are combined together on a receiving side, thus obtaining an effect produced by path diversity (or also referred to as transmission diversity) and thereby improving a transmission characteristic.
For example, the multipath-resistant modulation/demodulation scheme includes those in which improvements are made to modulation schemes, such as a spread spectrum scheme, an OFDM (Orthogonal Frequency Division Multiplexing) scheme in which information is transmitted while being distributed among a great number of sub-carriers arranged over a wide frequency range, an anti-multipath modulation scheme in which an anti-multipath property is exerted by providing a phase or amplitude redundancy in transmitted symbols, a PSK-VP (Phase Shift Keying with Varied Phase) scheme (non-patent document 1) in which a convex phase redundancy is provided, or a PSK-RZ (Return to Zero Phase Shift Keying) scheme in which an amplitude redundancy is provided (non-patent document 2), and those that use an ordinary modulation scheme but use an equalizer on the receiving side to exert an anti-multipath property.
For example, the spread spectrum scheme includes a DSSS (Direct Sequence Spread Spectrum) scheme in which an original signal is multiplied by a spread signal having a wider band than that of the original signal, a FHSS (Frequency Hopping Spread Spectrum) scheme in which a frequency is hopped over a wide band, and a THSS (Time Hopping Spread Spectrum) scheme in which a signal is spread with a wide band impulse.
In order to exert an active path diversity effect utilizing such a modulation/demodulation scheme with an anti-multipath property, there are conditions as follows with respect to an upper and lower limit of an arrival time difference between signals. Herein, the minimum and maximum, arrival time difference, with which a path diversity effect can be exerted, will be referred to as a delay resolution and a maximum delay, respectively. The delay resolution and the maximum delay may be determined based on the principle of the modulation/demodulation scheme used, or based on the parameters or limitations on implementation of the modulation/demodulation scheme.
For example, with the DSSS scheme, the delay resolution corresponds to a 1-chip length of a spread code while the maximum delay corresponds to an amount of time less than the spread code length. Therefore, when communicating with the DSSS scheme, it is possible, on the receiving side, to separate a received signal into delayed wave components and combine them together (RAKE reception) to obtain a path diversity effect as long as the arrival time difference is greater than or equal to the 1-chip length and less than the spread code length.
With the OFDM scheme, the delayed wave components are absorbed at, a guard interval set for the signal, whereby the maximum delay corresponds to a length of the guard interval. Inter-symbol interference does not occur if the arrival time difference between delayed waves is within the guard interval. Furthermore, since an error collection operation is performed over a plurality of sub-carriers, information can be reproduced even if some subcarriers have errors therein due to a multipath distortion. The delay resolution corresponds to a value around the inverse of the frequency bandwidth. Thus, with the OFDM scheme, it is possible to obtain the path diversity effect based on an effect, of the guard interval and a frequency diversity effect provided by scattering pieces of information over a wide frequency band and collecting the pieces together.
With the PSK-VP scheme or the PSK-RZ scheme, which is the anti-multipath modulation scheme, the delay resolution is greater than or equal to a value that is one-severalth of the symbol length while the maximum delay is an amount of time corresponding to less than one symbol length. Furthermore, even when a signal is transmitted utilising an ordinary single carrier scheme such as the PSK scheme or the QAM scheme, and the signal is then demodulated by using an equalizer with a tapped delay line on the receiving side, the delay resolution corresponds to a symbol length and the maximum delay corresponds to a temporal length determined by the number of taps.
An example of a wireless transmission system which uses such a modulation/demodulation scheme with an anti-multipath property so as to deliberately produce the path diversity effect, and improves the transmission characteristic will now be described.
Patent document 1 discloses a conventional wireless transmission system which performs communication utilising a modulation/demodulation scheme with an anti-multipath property. FIG. 51 is a block diagram of the wireless transmission system disclosed in patent document 1. FIG. 51 only shows a downstream path in which a signal is transmitted from a base station 310 to a mobile station 330. In FIG. 51, the base station 310 forms a communication area (wireless zone) 300 and communicates with the mobile station 330 within the area utilizing a CDMA (Code Division Multiple Access) scheme.
In the base station 310, a signal, outputted from a wireless unit 311 is transmitted to a relay unit 320 and the mobile station 330 via a transmitting antenna 312. In the relay unit 320, a signal S1 received by a receiving antenna 322 is delayed by a delay unit 324 so as to be inputted to a combiner 323. Also, a signal S2 received by an antenna 321 is directly inputted, to the combiner 323. The combiner 323 combines the signal S1 and the signal 82. A signal combined by the combiner 323 is amplified by the amplifier 325 so as to be transmitted to the mobile station 330 via a transmitting antenna 326.
The mobile station 330 is a RAKE receiver and receives three signals: a signal delayed by the relay unit, a signal not delayed by the relay unit, and a signal transmitted from the transmitting station. In the relay unit 320, since the delay unit 324 gives the signal S1 a delay greater than or equal to a code time period (chip length) of a spread code sequence, whereby a delay greater than or equal to the 1-chip length is generated among a plurality of signals. Then, the signals are received on the receiving side in the RAKE reception, thereby obtaining the path diversity effect and thus improving the transmission characteristic. The wireless transmission system deliberately provides an additional transmission path/delayed wave as described above, aiming at increasing the path diversity effect and improving the transmission characteristic.
Furthermore, in a modulation scheme, used in a transmission method disclosed in patent document 2, which is focused on a symbol waveform (phase waveform during a symbol), a phase of the symbol waveform synchronized with a symbol period T has a convex phase transition, and a detection output is obtained by performing delay (differential) detection, thereby eliminating a state where the detection output disappears due to a multipath. Furthermore, with an effect produced by combining the multipath, even the transmission characteristic can be improved. This improved effect is fundamentally exerted when a delay amount τ of a delayed wave is within a predetermined range (0<τ≦T).
FIG. 52 is a schematic diagram illustrating a phase transition of the symbol waveform disclosed in patent document 2. In the phase transition shown in FIG. 52, a transition width within a time length T (a symbol length) corresponding to one symbol is set up to a maximum phase transition amount φMAX, and a phase is changed in a parabolic shape based on a function shown by the following formula (1).φ(t)=(4φMAX/T2)·t·(T−t);(0<t<T)  (1)
FIG. 53 is a diagram illustrating a structure of a transmission signal generating circuit 700 disclosed in patent document 2. As shown in FIG. 53, the transmission signal generating circuit 700 includes a differential encoding circuit 701, a waveform generating circuit 702, an orthogonal modulator 704 and an oscillator 703. The transmission signal generating circuit 700 causes the differential encoding circuit 701 to differentially encode transmission data, the waveform generation circuit 702 to modulate the differentially encoded data by means of a symbol waveform having a convex phase redundancy, and the orthogonal modulator 704 to convert the modulated data into a signal having a carrier wave frequency band.
Next, a phase relationship between arriving signals obtained when using a symbol waveform having such a convex phase redundant waveform will be described.
FIG. 54 is a schematic diagram illustrating a phase relationship between two arriving signals A and B obtained when using the symbol waveform having the convex phase redundancy. In FIG. 54, it is assumed that the phase difference α is 180 degrees. In this case, even when there is a delay between the arriving signals, phases of the respective arriving signals are shifted in a convex manner. Therefore, within an effective segment (a segment in which correct received data can be obtained), while there is a segment in which the arriving signals may cancel each other out and a received wave may disappear (a point b of FIG. 54), there is also another segment in which the arriving signals do not cancel each other out and the received wave remains (a point a or c of FIG. 54). The arriving signals A and B are processed by means of a combination of delay (differential) detection and a low-pass filter, thereby making it possible to obtain an effective detection output. As a result, the transmission characteristic can be improved by obtaining a path diversity effect.
FIG. 55 is a schematic diagram illustrating a structure of a conventional wireless transmission system which uses the transmission diversity based on a modulation scheme disclosed in patent document 2. As shown in FIG. 55, a delay unit 901 is provided among the transmission signal generating circuit 700, a first antenna 904 and a second antenna 905, thereby interposing a delay between signals to be transmitted from the first antenna 904 and the second antenna 905. In this case, a delay amount interposed between the signals to foe transmitted is set such that the path diversity effect is well obtained. Thus, the transmission characteristic can be improved.    [patent document 1] Patent Application No. 2764150    [patent document 2] Patent Application No. 2506748    [non-patent document 1] H, Takai, “BER Performance of Anti-Multipath Modulation Scheme PSK-VP and its Optimum Phase-Wave form”, IEEE, Trans. Veh. Technol., Vol. VT-42, November 1993, p 625-640    [non-patent document 2] S. Ariyavisitakul, S. Yoshida, F. Ikegami, K. Tanaka, T, Takeuchi, “A Power-efficient linear digital modulator and its application to an anti-multipath modulation PSK-RZ scheme”, Proceedings of IEEE Vehicular Technology Conference, June 1987, p 66-71